1. Field of the Invention
The present invention is an enhanced type of fin for a serrated finned tube. The enhancement consists of increasing the fin's heat transfer capability by increasing the surface area of the segments provided on the fin. Enhancement may be performed either prior to or after serration of the fin into segments. The enhancement of the present invention may be accomplished either by impressing, cutting, flattening, rolling or otherwise providing indentations into the segments, thus broadening the segments and increasing their surface area.
2. The Prior Art
Finned tubes are employed in a process heater or boiler. Finned tubes are used because the fins on the tubes increase the exterior surface area of the tubes and thus increase their heat transfer capability. The function of the finned tubes is to transfer heat from hot flue gases located outside the finned tubes to a liquid, generally high purity water or a hydrocarbon, circulating inside the finned tubes. The heated liquid is used to operate a turbine or used for other process purposes.
Because of the high cost of fuel required to heat the liquid, it is important that transfer of thermal energy, i.e. heat, through the finned tube be as efficient as possible so the amount of fuel can be reduced. When the number of BTU's of fuel needed to heat the liquid is reduced, operating costs are significantly reduced, also. For these reasons finned tubes having large exterior surface areas are desirable.
The exterior surface areas of prior art finned tubes have been increased by at least two means, spacing the fins closer together and providing higher fins.
First, the fins of prior art finned tubes are attached to a pipe helically with adjacent helical spirals of the fins spaced apart. By spacing the fins closer together, more fins, and thus more surface area, can be attached to the tube per unit surface area of the tube, thus increasing the effective surface area of the tube.
However, if adjacent spirals of fins are spaced too closely together, space between adjacent fin spirals can plug up or become fouled. Fouling is dependent on the type of fuel which is burned. The resulting inadequate flow of flue gas between the fin spirals decreases their ability to absorb thermal energy from the flue gas. Also, if spaced still closer together, adjacent fins touch each other, thus decreasing their effective surface area with a resulting decrease in heat absorption efficiency. Providing adequate spacing between the spirals of prior art finned tubes thus limits the amount of exterior surface area attainable on a finned tube solely by means of spacing the fins closer together.
Second, the fins of prior art finned tubes are increased in height so that they extend outward further away from the tube, thus increasing the fin height and increasing surface area of the finned tubes. Increasing the fin height is more costly due to the additional material needed to produce the higher fin and due to the additional costs associated with transporting a larger and heavier finned tube or in transporting a larger and heavier heat exchanger produced from the larger finned tubes. Space constraints associated with the applications where the finned tubes will be employed often dictate the maximum allowable fin height, thus precluding an increase in fin height.
Higher fin segments are also weaker structurally, and they present more adverse conditions for interfin gas penetration. Also, the incremental surface generated by increasing the fin height is less and less effective, as compared to base tube surface and, therefore, is less cost effective because the lower fin efficiencies tend to negate some of the surface area gain. With lower fin efficiencies comes an increase in fin tip operating temperatures requiring the fin to be produced from more costly, higher heat resistant materials.
The present invention provides a cost effective way to increase the surface area of a serrated fin without spacing the fins closer together and without increasing the fin height. The present invention increases the surface area of segments of a serrated fin, either prior to or after serration, by causing the segments to be broadened, thus filling in a portion of each of the gaps which are formed between the segments in the normal process of serrating and forming the fins helically around the tube. The segments are broadened by impressing, cutting, flattening, rolling or otherwise providing an indentation, multiple indentations, or a pattern of indentations onto either part or all of the segment's surfaces.